Electronic assembly, and apparatus and method for the assembly thereof

ABSTRACT

An electronic assembly having a first layer and a second layer is disclosed. The first layer has a first interface surface and a plurality of cavities formed in the first interface surface. The second layer has a second interface surface and a plurality of projections disposed at the second interface surface, where the plurality of projections are aligned with and disposed at the plurality of cavities. An electrically conductive connecting material is disposed at the plurality of cavities such that the connecting material connects the plurality of projections to the respective plurality of cavities.

BACKGROUND OF THE INVENTION

The present disclosure relates generally to an electronic assembly andto the apparatus and method for assembling the electronic assembly, andparticularly to an apparatus and method for assembling a light detectorfor use in medical diagnostic equipment.

A type of detector array used for computed tomography is made from asilicon wafer having a diode array that is positioned on a ceramicsubstrate, such as Aluminum Nitride or AlN for example. The silicondiode-array wafer is connected, by means of stud-bump arrays, to metalpads on the ceramic substrate. The connection method may involve bondingvia a metal-filled adhesive, such as silver-filled epoxy for example, orby soldering. For backlit photodiode arrangements, the silicon wafer forthe diode array needs to be thin, so as to minimize the adverse effectson the X-ray beam that is converted into visible light via ascintillator for detection by the diode array. The thinner the silicondiode-array wafer is, the less attenuation and scattering there will beof the X-ray beam, however, the more susceptible the wafer will be tophysical damage. Additionally, for high resolution imaging, it isdesirable to have a silicon diode-array wafer that has a high density ofphotodiodes; however, such high density may require a high degree ofcontrol in the manufacturing of such detectors. While some advances havebeen made in the assembly of detector arrays for use in computedtomography, such as the use of vacuum suction cups to pick and place thediode array wafers, there is still a need in the art for a detectorarray arrangement, and a method and apparatus for assembling thedetector array arrangement, that overcomes these drawbacks.

BRIEF DESCRIPTION OF THE INVENTION

Embodiments of the invention include an electronic assembly having afirst layer and a second layer. The first layer has a first interfacesurface and a plurality of cavities formed in the first interfacesurface. The second layer has a second interface surface and a pluralityof projections disposed at the second interface surface, where theplurality of projections are aligned with and disposed at the pluralityof cavities. An electrically conductive connecting material is disposedat the plurality of cavities such that the connecting material connectsthe plurality of projections to the respective plurality of cavities.

Other embodiments of the invention include an apparatus for assemblingan electronic assembly having a top layer. The apparatus includes aporous rigid element having a thickness and a support surface, and ahousing configured to hold the porous rigid element and to provide apositive vacuum to the porous rigid element, wherein the appliedpositive vacuum results in a positive vacuum at the support surface forpicking up the top layer.

Further embodiments of the invention include a method for assembling anelectronic assembly, the assembly including a first layer having a firstinterface surface and a plurality of cavities formed in the firstinterface surface, and a second layer having a second interface surfaceand a plurality of projections disposed at the second interface surface.The first layer is positioned and the second layer is vacuum held via anapparatus such that the first and the second interface surfaces opposeeach other. The plurality of projections are aligned and engaged withthe plurality of cavities, and the vacuum hold is sufficiently reducedso as to release the second layer.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring to the exemplary drawings wherein like elements are numberedalike in the accompanying Figures:

FIG. 1 depicts an isometric view of an exemplary assembly for practicingembodiments of the invention;

FIG. 2 depicts a cross section side view of an expanded portion of theassembly of FIG. 1;

FIG. 3 depicts a similar view to that of FIG. 2, but prior to theassembly of the respective parts; and

FIG. 4 depicts an exemplary apparatus for assembling the assembliesdepicted in FIGS. 1-3.

DETAILED DESCRIPTION OF THE INVENTION

An embodiment of the invention provides a light detector for use inmedical diagnostic equipment, such as computed tomography for example,having an array of backlit photodiodes electrically bonded to a ceramicsubstrate having copper runs for signal communication. At the bondinginterface, the diode arrays have elongated electrical connections, orstud bumps as they are referred to, that extend into cavities formed inthe ceramic substrate, which provide pockets for constraining theconductive epoxy or solder from electrically shorting out adjacentdiodes. An alternative embodiment of the invention provides an apparatusfor vacuum holding the photodiode array while assembling the same ontothe ceramic substrate. A further embodiment of the invention provides amethod for using the assembly apparatus for assembling the lightdetector. While embodiments described herein depict an array of backlitphotodiodes assembled to a ceramic substrate as an exemplary electronicassembly, it will be appreciated that the disclosed invention is alsoapplicable to other electronic assemblies, such as processing chips on aprinted circuit board for example.

FIGS. 1-3 depict an exemplary embodiment of an electronic assembly 100having a first layer 110 and a second layer 120. FIG. 1 depicts anisometric perspective of electronic assembly 100, FIG. 2 depicts a sideview of a portion of the assembly of FIG. 1 subsequent to assembling,and FIG. 3 depicts a side view of the assembly of FIG. 1 prior toassembling. First layer 110 includes a first interface surface 112having a plurality of cavities 114 formed therein, and second layer 120includes a second interface surface 122 having a plurality ofprojections 124 disposed thereat. In an embodiment, second layer 120includes a diode array having a plurality of backlit photodiodes 121 inelectrical communication with projections 124. During the assembly ofsecond layer 120 onto first layer 110, projections 124 are aligned withand assembled into cavities 114. In an embodiment, the ends ofprojections 124 are shaped to mirror the shape of the interior surfaceof cavities 114, which may be spherical in shape for example.Projections 124 are electrically bonded to cavities 114 via anelectrically conductive connecting material 130, which may be conductiveepoxy or solder for example. Prior to assembly, connecting material 130may be applied to projections 124 or cavities 114, and in the assembledstate is referred to as being disposed at cavities 114. By introducingcavities 114 at first interface surface 112 of first layer 110,connecting material 130 will naturally be constrained by the pocketshape of cavities 114 during the curing of connecting material 130,which may involve the heating of an epoxy or the heating and cooling ofa solder, via a reflow and solidification process. Copper conductors 140at first layer 110 are exposed via an etching process that createscavities 114, thereby providing a point of electrical contact forconnecting material 130 to bond to, which in turn provides an electricalcommunication path from second layer 120 (a backlit photodiode array forexample), to first layer 110 (a ceramic substrate having wire runs forexample), and ultimately to readout electronics (not shown) for thediode signals. Copper conductors 140, also referred to as runs, may havelayers of nickel and gold plated thereon after cavities 114 are formed,thereby protecting the copper pads at cavities 114 from oxidation. As aresult of an etching process to create cavities 114, copper conductors140 may have a nominal thickness that is reduced slightly at the site ofcavities 114.

Cavities 114 are formed having a depth d with respect to first interfacesurface 112, and in the assembled state first interface surface 112 isdisposed apart from second interface surface 122 by a gap g, depicted inFIGS. 2 and 3. Accordingly, projections 124 have a length h that isequal to or less than the sum of depth d and gap g. If length h is lessthan the sum of depth d and gap g, such as may be the case under normaltolerance conditions, then connecting material 130 will bridge the gap.

Projections 124 have a width, or alternatively a diameter, w and arearranged with a pitch p. In an embodiment, projections 124 have a widthw equal to or greater than about 100 microns (micrometers) and equal toor less than about 700 microns. In an alternative embodiment,projections 124 have a width w equal to about 500 microns. In anembodiment, projections 124 have a pitch p equal to or greater thanabout 1.1 times width w and equal to or less than about 3 times width w.In an alternative embodiment, projections 124 have a pitch p equal toabout 2 times width w.

In an embodiment, and by appropriately sizing width w and pitch p, theplurality of photodiodes 121 in photodiode array 120, depicted in FIG.1, may be spaced on first layer 110 with an edge spacing s equal to orless than about 100 microns. In an alternative embodiment, edge spacings is equal to or less than about 25 microns, and in a furtherembodiment, edge spacing s is equal to about 10 microns. The closespacing between the photodiodes 121 of photodiode array 120 is madepossible at least in part by cavities 114 constraining connectingmaterial 130 so that adjacent photodiodes 121 are not electricallyshorted.

Referring now to FIG. 4, an apparatus 150 for assembling electronicassembly 100 is depicted in cross section view. In electronic assembly100, second layer 120 is also herein referred to as a photodiode arrayor a top layer, where top layer may be just one photodiode 121 ofphotodiode array 120. Apparatus 150 includes a porous rigid element 160having a thickness t and a support surface 162, and a housing 170configured to hold porous element 160 and to provide a positive vacuumto porous element 160 via a vacuum port 172. By applying a vacuum in thedirection of arrow 174, a positive vacuum results at the exposedsurfaces of porous element 160, and particularly at support surface 162,which is used for holding or picking up top layer 121. In an embodiment,housing 170 includes a pocket 176 with sides disposed around all sidesof porous element 160 except support surface 162, thereby leaving aportion of the thickness of porous element 160 exposed and directing thepositive vacuum at porous element 160 toward support surface 162.

In an embodiment, support surface 162 is fabricated to have a flatnessequal to or less than about 20 microns, that is, support surface 162defines a theoretical surface that does not vary from a planar surface,which contacts the theoretical surface at three or more points, by morethan 20 microns over the expanse of support surface 162. In analternative embodiment, support surface 162 is fabricated to have aflatness equal to or less than about 10 microns. Porous element 160 maybe fabricated with the desired flatness at support surface 162 with orwithout secondary machining.

As depicted in FIG. 4, support surface 162 may have an overall dimensionthat is substantially matched to the corresponding overall dimension ofphotodiode 121, as evidenced by only a slight overhang 164 at the edgeof porous element 160. While the cross section view of Figure 4 showsonly one overall dimension of support surface 162 being substantiallymatched to the corresponding overall dimension of photodiode 121, namelythat overall dimension in the plane of the paper, it will be appreciatedthat a second overall dimension of support surface 162 may also besubstantially matched to that corresponding overall dimension ofphotodiode 121, namely the overall dimension perpendicular to the planeof the paper.

Porous element 160 may include a plurality of cavities 166 havingdisposed therein a plurality of heater elements 180 for processingconnecting material 130 so as to adhere the projections 124 to cavities114. In an embodiment, the processing of connecting material 130involves heating electrically conductive epoxy to cure and solidify theepoxy. In an alternative embodiment, the processing of connectingmaterial 130 involves heating solder to cause a solder re-flow thatsolidifies on cooling. Heating elements 180 are in signal communicationwith a control unit (not shown) for controlled heating during assembly.The control unit also controls the presence and absence of a vacuum atvacuum port 172, and the action of control arm 190 that is in operablecommunication with apparatus 150 for controlling the location ofapparatus 150 during assembly. The control unit may be any type ofcontrol unit suitable for the purposes disclosed herein.

Since photodiodes 121 may be manufactured with a less than desirableflatness, apparatus 150 is configured such that the combination ofsupport surface 162 and resultant positive vacuum at support surface 162is sufficient to flatten photodiode 121, having an original flatnessequal to or greater than about 50 microns and a typical thickness equalto about 100 microns, to a final flatness equal to or less than about 20microns. In an alternative embodiment, the combination of supportsurface 162 and resultant positive vacuum at support surface 162 issufficient to flatten photodiode 121, having an original flatness equalto or greater than about 100 microns, to a final flatness equal to orless than about 10 microns.

In view of the foregoing, a method of assembling electronic assembly 100using apparatus 150 includes: positioning first layer at an assemblystation (not shown); vacuum holding second layer 120, 121 via apparatus150 such that first and second interface surfaces 112, 122 oppose eachother; aligning plurality of projections 124 with plurality of cavities114; engaging plurality of projections 124 with plurality of cavities114; and, sufficiently reducing the vacuum hold at support surface 162so as to release second layer 121 from apparatus 150. An electricallyconductive connecting material 130 may be applied to the plurality ofcavities 114 or to the plurality of projections 124 prior to engagingprojections 124 with cavities 114. Subsequent to the engaging action,the connecting material 130 is processed in the manner discussedpreviously so as to adhere projections 124 to cavities 114. In anembodiment, the vacuum hold may be maintained for a sufficient amount oftime to allow connecting material 130 to at least partially cure enoughto keep photodiode 121 in a firm position.

The method disclosed herein may enable accurate placement of photodiodes121 to within a sidewise tolerance of about 10 microns, and to aparallel tolerance, with respect to the ceramic substrate 110, of about30 microns.

In an alternative embodiment, and with reference to Figures 2 and 4,projections 124 may be of uniform thickness, as depicted in FIG. 2, orof non-uniform thickness, as depicted in FIG. 4. Where projections 124are non-uniform in thickness, a wide base 125 may be utilized with aconductive pad 126 for improved signal communication between photodiode121 and first layer 110.

As disclosed, some embodiments of the invention may include some of thefollowing advantages: a high packing density of photodiodes on theceramic substrate with a small spacing therebetween; effectivecontainment of the conductive connecting material to prevent shortingbetween adjacent diodes; stud bumps of a photodiode having a small pitchwith respect to the dimension of the photodiode; ability to flatten thephotodiode during assembly by using a pick-and-place apparatus having avacuum across the support surface; and, use of heater elements integralwith the assembly apparatus for enhanced productivity in curing theepoxy or solder.

While the invention has been described with reference to exemplaryembodiments, it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted forelements thereof without departing from the scope of the invention. Inaddition, many modifications may be made to adapt a particular situationor material to the teachings of the invention without departing from theessential scope thereof. Therefore, it is intended that the inventionnot be limited to the particular embodiment disclosed as the best oronly mode contemplated for carrying out this invention, but that theinvention will include all embodiments falling within the scope of theappended claims. Moreover, the use of the terms first, second, etc. donot denote any order or importance, but rather the terms first, second,etc. are used to distinguish one element from another. Furthermore, theuse of the terms a, an, etc. do not denote a limitation of quantity, butrather denote the presence of at least one of the referenced item.

1. An electronic assembly comprising: a first layer having a firstinterface surface and a plurality of cavities formed in the firstinterface surface; a second layer having a second interface surface anda plurality of electrically conductive projections disposed at thesecond interface surface, wherein the plurality of projections arealigned with and disposed at the plurality of cavities, the second layercomprising a diode array having a plurality of backlit photodiodes inelectrical communication with the plurality of projections; and anelectrically conductive connecting material disposed at the plurality ofcavities such that the connecting material non-separably connects theplurality of projections to the respective plurality of cavities, eachof the plurality of cavities being configured to constrain theconnecting material disposed thereat from outward flow; wherein theplurality of cavities are formed having a depth d in the first interfacesurface; wherein the first interface surface is disposed part from thesecond interface surface by a gap g; and wherein the plurality ofprojections have a length h that is equal to or less than the sum of thedepth d and the gap g, such that the connecting material bridges thedistance defined by (d+g−h).
 2. (canceled)
 3. The assembly of claim 1,wherein: the plurality of projections have a width w equal to or greaterthan about 100 microns and equal to or less than about 700 microns. 4.The assembly of claim 3, wherein: the plurality of projections have awidth w equal to about 500 microns.
 5. The assembly of claim 3, wherein:the pitch of the plurality of projections is equal to or greater thanabout 1.1 times the width w and equal to or less than about 3 times thewidth w.
 6. The assembly of claim 5, wherein: the pitch of the pluralityof projections is equal to about 2 times the width w.
 7. The assembly ofclaim 1, wherein: the plurality of projections are shaped to mirror theshape of the plurality of cavities.
 8. The assembly of claim 1, wherein:the first layer comprises a ceramic substrate; and the connectingmaterial comprises a conductive epoxy, a conductive solder, or anycombination comprising at least one of the foregoing materials. 9.(canceled)
 10. The assembly of claim 1, wherein adjacent projections areabsent direct electrical communication.
 11. The assembly of claim 8,wherein: the assembly comprises a light detector for use in medicaldiagnostic equipment.
 12. The assembly of claim 8, wherein: theplurality of photodiodes are spaced on the first layer with an edgespacing equal to or less than about 100 micrometers.
 13. The assembly ofclaim 12, wherein: the plurality of photodiodes are spaced on the firstlayer with an edge spacing equal to or less than about 25 micrometers.14. The assembly of claim 13, wherein: the plurality of photodiodes arespaced on the first layer with an edge spacing equal to about 10micrometers. 15-34. (canceled)
 35. An electronic assembly comprising: afirst layer having a plurality of pockets; a second layer comprising adiode array having a plurality of backlit photodiodes having a pluralityof electrically conductive projections, wherein the plurality ofprojections are aligned with and disposed at the plurality of pocketswith a defined distance therebetween; and an electrically conductiveconnecting material disposed at the plurality of pockets such that theplurality of projections are electrically and non-separably bonded tothe respective plurality of pockets via the electrically conductiveconnecting material; wherein the conductive connecting material bridgesthe defined distance; and wherein each of the plurality of pockets areconfigured to constrain the connecting material disposed thereat fromoutward flow.
 36. A multi-layer backlit photodiode array electronicassembly, comprising: a first layer having a plurality of pockets formedin a first interface spice; a second layer having a plurality ofelectrically conductive projections disposed at a second interfacesurface, wherein the plurality of projections are aligned with anddisposed at the plurality of cavities, and wherein the plurality ofections and the plurality of pockets are spaced apart by a defineddistance; and an electrically conductive connecting material disposedbetween the plurality of pockets and the plurality of projections suchthat the connecting material bridges the defined distance andnon-separably connects the plurality of projections to the plurality ofpockets, each of the plurality of pockets being configured to constrainthe connecting material disposed thereat from outward flow; wherein thesecond layer comprises the backlit photodiode army.